'When two or more diseases occur at the same time, it is referred to as 'comorbidity', said AIHW spokesperson Louise York.

'Sometimes these diseases occur together simply by chance, but often it's because there are some associations between them, such as shared risk factors.'

Ageing is a factor that has a particularly strong association with comorbidity.

'Older people are more vulnerable to developing many diseases, and Australians' increasing life expectancy means a greater chance for multiple conditions to arise,' Ms York said.

Nearly 40% of Australians aged 45 and over have two or more of the eight chronic diseases examined in today's release.

'For this age group, the two most common chronic diseases to occur in combination with any other chronic disease were arthritis and cardiovascular disease,' Ms York said.

'When looking at particular combinations of diseases in this age group, we found that arthritis and cardiovascular disease occurred together most frequently, in 16% of the population, followed by arthritis and back problems (10%) and back problems and cardiovascular disease (8%).'

Among the younger age group (0-44 years), mental health conditions and back problems were the most common comorbidities, followed by mental health and asthma, and back problems and asthma.

'Comorbidities are associated with poorer health outcomes, more frequent use of health services, and higher healthcare costs,' Ms York said.

'But studying comorbidities allows preventive, management and treatment services to be better planned to meet the needs of those affected.'